Emulsions are mixtures of at least two components which are substantially immiscible in each other, and a surfactant which lowers interfacial tension between the two phases. A microscopic view of aqueous emulsions reveals two phases, an oil phase and a water phase. Depending upon the proportion of each component, the emulsion can be characterized as an oil-in-water emulsion or a water-in-oil emulsion. The chief distinction between the two being which component, the oil or water phase, comprises the continuous portion of the emulsion. The noncontinuous phase is in the form of droplets in the other phase.
Standard emulsions are opaque like milk due to the particle sizes of the droplet phase, and the difference in refractive indices between the oil and aqueous phases. Standard emulsions of polydiorganosiloxane are used in the formation of protective coatings and release coatings, as masonry impregnants, anti-foaming agents, leather treatments, and emollients in cosmetic applications.
Methods for making standard aqueous emulsions of polydiorganosiloxanes are well known in the art and can be separated into two types--mechanical means and emulsion polymerization means. Mechanical means typically involve homogenizing a mixture of polydiorganosiloxane, surfactant, and water using millingmachinery to obtain the desired droplet sizes. Mechanical methods have limitations. Since mechanical methods of formation require physical work, only polydiorganosiloxanes of limited viscosity can be used to make standard emulsions. In the case of high viscosity fluids and resins, the fluid or resin must be dissolved in a water immiscible solvent, which is then homogenized with the aqueous phase. Since emulsions are employed to avoid using the verY solvents used to dissolve the fluids and resins, such methods are considered undesirable.
A mechanical method for making standard, opaque emulsions of polydiorganosiloxane is described in U.S. Pat. No. 2,755,194 issued to Volkmann, Feb. 7, 1955. The method involves mixing a surfactant with polydimethylsiloxane with a viscosity of 350 cs. at 25.degree. C., adding a small amount of water to the mixture, mixing in a colloid mill, sequentially adding water and remilling until the desired amount of water is present in the emulsion. This method does not produce clear polydimethylsiloxane oil-in-water microemulsions.
Emulsion polymerization methods for making emulsions of high viscosity polymers involve starting with low viscosity polymer precursors, i.e., monomers, or reactive oligomers, which are immiscible in water, a surfactant to stabilize the polymer precursor droplet in water, and a water soluble polymerization catalyst. These components are added to water, the mixture is stirred and polymerization is allowed to advance until the reaction is complete or the desired degree of polymerization is reached and a standard emulsion of the polymer is formed.
An example of an emulsion polymerization is taught in U.S. Pat. No. 2,891,920 issued to Hyde et al., which shows a method for making aqueous emulsions of polydimethylsiloxane starting with precursor molecules of the polydimethylsiloxane. Standard emulsions of polyorganosiloxane and water have a number of shortcomings, particularly, their stability to remain emulsions with the passage of time, freeze-thaw cycling, and their milky, or opaque appearance.
Microemulsions are mixtures of oil and water where the particle size of the resulting droplets is small enough so the resulting mixture is clear. Because of their clarity microemulsions are distinguishable form standard, opaque emulsions. Microemulsions of polydiorganosiloxane and water offer a number of advantages over standard emulsions. The clarity of the mixtures is advantageous in cosmetic applications, and the reduced particle size of the droplets is advantageous where it is necessary to deposit particles in small pores, for instance, in leather treatment processes. Microemulsions are also more temperature, dilution, and formulation stable than standard emulsions
Methods for making microemulsions of polydiorganosiloxane are known in the literature, however, the methods for making these have limitations that severely hamper their usefulness.
U.S. Pat. No. 3,433,780 issued to Cekada (Mar. 18, 1982) teaches a method for making colloidal suspensions of silsesquioxanes with the unit formula RSiO.sub.3/2 where R is a hydrocarbon radical. Silsesquioxanes are materials with three Si--O bonds per silicon. Cekada's method results in particle sizes of 10 to 1000A, silicone materials content of about 10%, and does not teach how to make polydiorganosiloxane microemulsions.
U.S. Pat. Nos. 3,975,294 and 4,052,331, both issued to Dumoulin, teach methods for making microemulsions comprised of polydimethylsiloxane, water, and a special mixture of emulsifying agents claimed in the earlier issued patent. Dumoulin mixes polydimethylsiloxane, water, and the special emulsifier, and then homogenizes the mixture. Since this is a mechanical means of making an emulsion, the siloxane oils used in the process are by necessity low molecular weight (the examples use oils with viscosities of no more than 585 cP, which have molecular weight of 10,000 to 20,000). Otherwise, Dumoulin shows that his emulsifier makes standard emulsions of higher molecular weight oils. The microemulsions of Dumoulin are undesirable in processes where deposition of polysiloxane is desired because of their high surfactant content, and their relatively low polydiorganosiloxane content. Also, Dumoulin teaches aqueous polydiorganosiloxane microemulsions using a specific surface active composition. Variation of his emulsifier composition produces standard opaque emulsions.
U.S. Pat. No. 3,975,294, issued to Rosano, Aug. 17, 1976, teaches a method for making microemulsions which comprises: choosing a surfactant which is slightly more soluble in the oil than in the aqueous portion of the proposed emulsion; adding the chosen surfactant to the oil phase to form a clear solution; adding the resulting mixture to an aqueous solution containing a second surfactant which is more soluble in the water than in the oil; and then agitating. Rosano specifically teaches that the two surfactants required in the practice of his method must be different. It is important that the two surfactants have different solubility properties.
Rosano claims two methods for making microemulsions of polydiorganosiloxane and water, both of which have a step of dissolving the polydiorganosiloxane in trichlorotrifluoroethane, and then adding the first surfactant to the resulting solution. No polymerization of the polydiorganosiloxane fluid is taught by Rosano, and his example of a polydiorganosiloxane, water, microemulsion has a high, more than 3 to 1 surfactant-to-oil ratio, and low polydiorganosiloxane content of less than 4% in the final microemulsion. Rosano's method has the limitations of typical mechanical methods.
Standard opaque emulsions of polydiorganosiloxane are well known in the prior art. Specific art pertinent to this application is disclosed in U.S. Pat. No. 2,891,920 issued to Hyde et al., June 23, 1959. Hyde teaches a method of combining a polysiloxane precursor (such as a cyclopolysiloxane, a hydroxy end-blocked siloxane oligomer, a dialkoxydialkylsilane, or a trialkoxyalkylsilane), surfactant, polymerization catalyst, and water to form a mixture, and heating the mixture while agitating to form an emulsion of the resulting high molecular weight siloxane and water. Hyde teaches that strong mineral acids and strong bases are catalysts for polymerizing cyclopolysiloxane into linear polysiloxanes in an emulsion polymerization process that yields stable, standard, aqueous emulsions.
U.S. Pat. No. 3,294,725 issued to Findley et al. teaches the use of various sulfonic acids as catalysts for the emulsion polymerization of cyclopolysiloxanes, siloxane oligomers, and monofunctional and trifunctional silanes such as methyltrimethoxysilane and dimethyldimethoxysilane. Findley teaches that aliphatically substituted benzenesulfonic acids, aliphatically substituted naphthalenesulfonic acids, aliphatic sulfonic acids, silylalkylsulfonic acids and aliphatically substituted diphenyl ether sulfonic acids act as polymerization catalysts for the emulsion polymerization of cyclopolysiloxanes and also act to stabilize the resulting standard emulsions of linear polysiloxanes by acting as surface active agents when the aliphatic portion of the acid is between 6 and 18 carbon atoms long.
The methods described in Hyde and Findley using cyclopolydiorganosiloxanes as starting materials give stable aqueous emulsions of polydiorganosiloxanes. Hyde teaches that fine particle sizes can be obtained by allowing greater degrees of polymerization to occur, but no examples are given which show clear microemulsions.
One of the objects of the present invention is to provide a method for making clear stable aqueous microemulsions of polydiorganosiloxane and water. Another object of this invention is to provide a method that is capable of making clear, stable aqueous microemulsions of polydiorganosiloxane which have surfactant to polydiorganosiloxane ratios of less than one.
Another object of the present invention is to provide a method for making clear, stable aqueous microemulsions which have average droplet sizes less than 0.15 micron.
Another object of the invention is to provide a method for making clear, stable aqueous microemulsions of polydialkylsiloxane which have high polydiorganosiloxane contents in the range of 10% to 35% by weight, and mean number molecular weights of the polydiorganosiloxane materials greater than 20,000.